Method of electrolysing manganous chloride in a diaphragm cell



United States Patent 3,477,925 METHOD OF ELECTROLYSING MANGANOUS CHLORIDE IN A DIAPHRAGM .CELL Coruelis Marius Kooi, Oust-Voorne, and Antienus Louissbn, Hengelo, Netherlands, assignors to N.V. Koninklijke Netlerlandsche Zoutiudus'trie, Hengelo, Netherlands, a Dutch company No Drawing. Filed Aug. 10, 1966, Ser. No. 571,392 Int. Cl. 022d 3/18, 1/24; B011: 3/10 US. Cl. 204-105 7 Claims ABSTRACT OF THE DISCLOSURE Metallic manganese and chlorine are produced in a diaphragm cell by electrolysis of an aqueous solution of MnCl and NH Cl as the catholytic and acidified MnCl free from ammonium ions, as the analyte.

This invention rel-ates to an improved method for the electrolytic preparation of manganese and chlorine in a diaphragm cell.

In practice, electrolytes containing manganous sulfate are used for electrowinning manganese. Though several advantages of the electrolysis of manganous chloride solutions over the manganous sulfate electrolysis have been appreciated for a long time, until now a commercially feasible manganous chloride electrolysis has not been realized.

The difiiculties encountered in this electrolysis are primarily connected with the reactions occurring at and around the anode. When using electrolytes or anolytes containing manganous chloride and having the compositions proposed in the literature, gaseous chlorine is not the main anodic product but other electrolysis or reaction products, in particular insoluble compounds of manganese in thetrior polyvalent state, are also, or even exclusively, obtained. When the electrolyte contains ammonium chloride in addition to manganous chloride, as is usually the case, chloramines and nitrogen may also be found among the products.

To avoid these difliculties, it has already been proposed to effect the electrolysis of manganous chloride in a diaphragm cellwhi'le using hydrochloric acid or an alkali chloride solution as the anolyte (see US. Patent 2,810,685). It is true that due to the absence of manganous and ammonium ions in the anolyte the'formation ofinsoluble compounds of manganese at the anode is avoided and the occurrence of undesirable reactions between the chlorine formed at the anode and ammonium ions is excluded but, on the other hand, this proposal has serious drawbacks preventing its practical realization.

Among the drawbacks of this proposal, the unavoidable migration of cations (hydrogen ions and alkali metal ions, respectively), from the anolyte through the diaphragm to the cathode compartment raises neutralization and/or removal problems which cannot be solved in an economically acceptable manner.

From published experiments effected with anolytes containing, in addition to hydrogen chloride, manganous chloride and ammonium chloride, it appeared that in us ing a manganous chloride concentration of 0.25 mole/ litre and an ammonium chloride concentration of 0.5 mole/litre an anodic current efliciency of 60% is obtained only when the hydrogen chloride concentration is raised to 6 moles/litre (see the publication of V. N. Lisov, V. N. Plakhotnik and V. V. Stender, Anodic Evolution of Chlorine in the Electrolysis of Hydrochloric Acid in the Presence of Salts of Ammonium and Manganese, in Zh. Prikl. Khim. 37 (1964), No. 7, 14984504, summarized in Chem. Abstracts 61 (1964), 10311). In this way, the problem of the hydrogen ion migration is not 3,477,925 Patented Nov. 11, 1969 ice solved at all since the fraction of the (noninterfering) manganous and ammonium ions in the flow of cations migrating to the cathode compartment is: negligible. Moreover, there is not only a current efiiciency loss of 40% but also a loss of ammonium ions whichare converted into nitrogen by oxidation.

It is, therefore, an object of the present invention to provide an improved method for the production of manganese metal and chlorine from man-ganous chloride.

It has now been found that, on the one hand, the hydrogen ion migration can be limited considerably and, on

' the other hand, excellent anodic current efficiencies can be obtained by using an anolyte containing substantially no ammonium ions and containing considerably greater amounts of manganous chloride and smaller amounts of hydrogen ions than used in the experiments mentioned hereinbefore.

In accordance with the above, this invention relates to a method of electrolysing manganese chloride in a diaphragm cell wherein manganese is deposited on the cathode from a catholyte containing manganous chloride and ammnoium chloride, and chlorine is evolved at the anode from an anolyte containing hydrogen and chlorine ions while avoiding the formation of insoluble compounds of manganese in the anolyte, and is characterized in that the anolyte is substantially free of ammonium ions and contains per litreat least 1 mole of manganous chloride and not more than 2.5 grarnions of hydrogen ions.

Due to this invention, undesirable reactions in the anode compartment can be avoided while simultaneously preventing the occurrence of an excessive migration of hydrogen ions to the cathode compartment.

Though the use of other acids is not excluded, generally, hydrochloric acidwill be used for providing the desired hydrogen ion concentration so as to avoid, the presence of anions other than chloride ions. In certain cases, it may be advantageous to add compounds other thanmanganous chloride and hydrogen chloride to the anolyte. For example, in case the catholyte contains potassium chloride for influencing the deposition of manganese onto the cathode, the anolyte may also contain a certain amount of potassium chloride.

The following results obtained with experiments carried out in the achievement of the present invention may serve for illustrating the particular fact, referred to hereinbefore, that theacid concentration of theanolyte can be lower as the unanganous chloride concentration is higher.

1 ml. of a solution of 0.02 mole of potassium perman-v ganate per litre was added to each of 25 ml. portions of solutions having different concentrations of manganous chloride and hydrogen chloride. Therebyjpart of the manganous ions present in the solutions was oxidized to manganic ions, just as occurs in electrolysing anolytes containing manganous chloride. In the following Table 1, there has been indicated in which solutions a precipitate was formed. i

TABLE 1 Formation of precipitate when addin 1 ml. of I sell t of KMnOa of 0.02 mole/litre to 2'5 ml. o f a solutl n ha vi g the given concentrations of MnCIa and H01 H01 (mole/litre) M11011: (mole/litre):

Formation of precipitate when adding 1 ml. of a solution of KMnQr of 0.1 mole/litre to 25 ml. of a solution having the given concentrations of M1101, and H01 HG] (mole/litre) In such a manner, it is possible to determine the minimum amount of hydrogen chloride appertaining to a predetermined manganous chloride concentration for each manganic ion concentration.

The manganic ion concentration in the diffusion layer of the anolyte at the anode depends on several factors, in particular the current density at the anode, the turbulence and rate of flow of the anolyte, the temperature and the anode material. Thus, at a given manganous chloride concentration, the hydrogen chloride concentration can be lowered when using a lower current density, a greater rate of flow or a higher temperature or when choosing platinum instead of graphite as the anode material.

As to the last mentioned factor, it appeared that at a total current efiiciency for the formation of chlorine and manganic ions of 100% the fraction of manganic ion formation is only a few percent when using a platinized titanium anode, of the order of 20% when using a platinum anode, and up to 70% or more when using a graphite anode.

This means, for example, that given a certain hydrogen ion migration which is deemed to be desirable or admissible, a greater current density can be applied when using a platinum anode than when using a graphite anode, the other conditions being the same.

Generally, an anolyte having a manganous chloride concentration of at least 1.5 mole/litre and a hydrogen ion concentration of not more than 1.5 gramion/litre will be used. Particularly advantageous operation conditions are obtained when the electrolysis is effected while using a noble metal anode and an anolyte containing 0.2-1.0 mole of hydrogen chloride per litre.

In particular when the electrodes and the diaphragm are positioned vertically and when using a diaphragm having a permeability usually encountered in practice, it is of interest in view of limiting the convection through the diaphragm that in the operating cell the apparent specific gravities of the catholyte and of the anolyte are not too different. As will appear from t l 1e examples, the requirement of having no great difference between the specific gravities can be met while maintaining good anodic and cathodic current efliciencies. In this case, generally, the anolyte contains not more than 3.2 moles of'manganous chloride per litre and the catholyte contains 1.2-2.5 moles of manganous chloride per litre.

When necessary, a conversion of electrolytically formed manganic ions and chloride ions into manganous ions and chlorine can be effected in a simple manner by heating the anolyte. Preferably, the anolyte is not heated within but outside the anode compartment to a temperature of at least 60% C. and thereafter cooled prior to being returned to the electrolysis cell so as to prevent the catholyte temperature from rising above the optimum for manganese deposition (generally 25-40 C.) by heat transfer from the anolyte to the catholyte.

When it is desirable to promote the said conversion, this cannot only be efiected by heating the anolyte but also by using a catalyst for the conversion, such as a compound of silver, platinum, lead or copper. Thus, the manganic ion concentration can be kept at a low level without raising the temperature by using a graphite electrode impregnated with silver chloride as the anode.

The following examplesmay serve to illustrate this invention.

EXAMPLE I In a laboratory cell having an iron cathode, a platinized titanium anode and a ceramic diaphragm the following electrolytes were electrolysed during 24 hours at a current density of 10.7 amperes per dm. a cell potential of 3.8 volts and a temperature of 30 C.

Catholyte:

190 g. (1.5 moles) of MnCl per litre 200 g. (3.7 moles) of NH Cl per litre pH 0.3 Specific gravity 1.18 Rate of How 2.5 l./h.

Anolyte:

400 g. (3.2 moles) of MnCl per litre 12.5 g. (0.34 mole) of HCl per litre Specific gravity 1.32 Rate of flow 2.5 l./h.

617 g. of manganese were deposited on the cathode, corresponding to a cathodic current eiiiciency of 78%. The anodic current efliciency was 94% EXAMPLE II In the same cell as used in Example I, but now provided with a woven polyester cloth as the diaphragm, the following electrolytes were electrolysed during 20 hours at a current density of 10 amperes per dm. a cell potential of 4.1 volts and a temperature of 30 C.

Catholyte:

194 g. (1.55 moles) of MnCl per litre g. (3.4 moles) of NH Cl per litre pH 0.3 Specific gravity 1.18 Rate of flow 3.3 l./h.

Anolyte:

300 g. (2.4 moles) of MnCl per litre 24.5 g. (0.67 mole) of HCl per litre Specific gravity 1.24 Rate of flow 3.3 l./h.

444 g. of manganese, corresponding to a cathodic current eflicrency of 72% were deposited on the cathode. The anodic current efficiency was 87%.

EXAMPLE III In a semicommercial cell having a copper cathode, a platinized titanium anode and a ceramic diaphragm 3500 g. of manganese were produced in 26 hours at a current density of 10.3 amperes per dm. a cell potential of 4.3 volts, a temperature of 33 C. and catholyte and anolyte flow rates of 16 l./h. while using the following electrolytes.

Catholyte:

200 g. (1.6 moles) of MnCl per litre 200 g. (3.7 moles) of NH Cl per litre Specific gravity 1.21 Anolyte:

400 g. (3.2 moles) of MnCl per litre 22 g. (.06 mole) of HCl per litre Specific gravity 1.32 The cathodic and anodic current efiiciencies were 69% and 75% respectively.

This invention may be variously modified and embodied within the scope of the subjoined claims.

What is claimed is:

1. An electrolytic method for the production of manganese metal and chlorine from manganous chloride which comprises passing an aqueous catholyte solution containing manganous chloride and ammonium chloride through the cathode compartment of an electrolysis cell having an anode and a cathode in separate compartments separated by a diaphragm, passing an aqueous anolyte solution being substantially free of ammonium ions and containing at least 1 mole of manganous chloride and not more than 2.5 gramions of hydrogen ions per litre through the anode compartment of the cell, and establishing electrical conduction through the cell by application of an electric potential to deposit manganese metal at the cathode and to discharge chlorine gas at the anode.

2. The method of claim 1 in which the said anolyte contains at least 1.5 moles of manganous chloride and not more than 1.5 gramions of hydrogen ions per litre.

3. The method of claim 1 in which said anode is a graphite anode impregnated with a catalyst for the conversion of manganic ions and chloride ions into manganous ions and chlorine.

4. The method of claim 3 in which said graphite anode is impregnated with silver chloride.

5. An electrolytic method for the production of manganese metal and chlorine from manganous chloride which comprises passing an aqueous catholyte solution containing manganous chloride and ammonium chloride through the cathode compartment of an electrolysis cell having an anode selected from the group consisting of noble metal anodes and noble metal coated anodes, and a cathode in separate compartments separated by a diaphragm, passing an aqueous anolyte solution being substantially free of ammonium ions and containing at least 1.5 moles of manganous chloride and from 0.2 to 1.0 gramion of hydrogen ions per litre through the anode compartment of the cell, and establishing electrical conduction through the cell by application of an electric potential to deposit manganese metal at the cathode and to discharge chlorine gas at the anode.

6. The method of claim 5 in which the said anolyte contains not more than 3.2 moles of manganous chloride per litre and the said catholyte contains from 1.2 to 2.5 moles of manganous chloride per litre.

7. An electrolytic method for the production of manganese metal and chlorine from manganous chloride which comprises passing an aqueous catholyte solution containing manganous chloride and ammonium chloride through the cathode compartment of an electrolysis cell having an anode and a cathode in separate compartments separated by a. diaphragm, introducing, withdrawing fortifying and returning an aqueous anolyte solution being substantially free of ammonium ions and containing at least 1 mole of manganous chloride and not more than 2.5 gramions of hydrogen ions per litre to the anode compartment so as to effect a conversion of electrolytically formed manganic ions and chloride ions into manganous ions and chlorine, and being cooled to a temperature below C. prior to being returned to said anode compartment, and establishing electrical conduction through the cell by application of an electric potential to deposit manganese metal at the cathode and to discharge chlorine gas at the anode.

References Cited UNITED STATES PATENTS 2,398,614 4/1946 Brandt et a1 204- 2,810,685 10/1957 Sakowski 204105 FOREIGN PATENTS 740,837 11/1955 Great Britain.

JOHN M. MACK, Primary Examiner H. M. FLOURNOY, Assistant Examiner US. Cl. X.R. 204-128 

